This invention relates to improved switching materials which exhibit Ovonic switching properties. Ovonic switching phenomena have been known and reported in the past for numerous thin film amorphous semi-conductors. The so-called Ovonic switching properties can be broken into two categories, namely, threshold switching and memory switching. Each has its particular uses in solid state physics and electronics.
Threshold switching refers to the phenomena in which a material acts as an insulator and blocks current until a voltage of just the right threshold is applied, at which point the material suddenly becomes a conductor and passes current and the voltage sustained across the sample decreases. The device continues to pass current as long as the required voltage is applied, but when the voltage drops below a critical value, referred to as the holding voltage, the device again goes into the resistance or blocking state.
Contrasted with "threshold switching" is "memory switching". A memory switching device also switches from blocking to conducting when a threshold voltage is applied, but it then remains in the conducting state even when the current is turned off. In other words, it remembers the "on" state. Thus, the device has a memory of the last state that it was in.
The threshold switching phenomena and the memory switching phenomena have been known and both have been exhibited by thin film, or amorphous semi-conductors, first discovered by Mr. Stanford R. Ovshinsky. However, thin film amorphous semi-conductors are difficult to produce, require highly skilled labor and much expense in their production, and are limited in usage because they can dissipate only a limited amount of power. Thus, thin film amorphous semi-conductors have often been limited for potential commercial use.
There is therefore a real and continuing need for switching materials other than the known thin film amorphous semi-conductors. Accordingly, it is a primary objective of the present invention to prepare switching materials, in bulk form, which are polycrystalline ceramic materials which are capable of exhibiting threshold switching or memory switching, depending upon the precise composition.
Another objective of the present invention is to develop a new and unique switching material which is a polycrystalline ceramic composed, primarily, of zinc oxide plus additives of other inorganic oxides.
Another objective of the present invention is to prepare polycrystalline ceramic switching materials which can be prepared by standard ceramic processing methods, such as dry pressing and firing.
Yet another objective of the present invention is to prepare a switching material by standard ceramic processing methods which can be prepared in bulk form so that they can act not only as switching materials, but can also simultaneously serve as a substrate or a mechanical support material in an electrical or physical-electrical device.
Another objective of the present invention is to prepare a switching material, which can be either a threshold switching material or a memory switching material, which is made up of rather stable oxides, such that sintering can be done in an atmosphere of air, so that atmospheric control, such as is required in preparing thin film amorphous semi-conductors, is not necessary.
An even further objective of the present invention is to prepare ceramic switching materials which display either memory switching or threshold switching, depending upon the composition of the other inorganic oxides mixed with the major portion of zinc oxide.
A still further objective is to prepare the first ever, polycrystalline ceramic material which displays threshold switching.
Another objective is to prepare switching materials which, unlike previously produced amorphous semi-conductor materials, can be formed by ceramics in bulk, rather than formed as fragile, difficult to produce, and expensive thin film glasses, or single crystal layer compounds such as stannous sulfide or zirconium sulfide.
The method and manner of accomplishing each of these objectives as well as others, will become apparent from the detailed description of the invention which will follow hereinafter.